MEASUREMENT SYSTEMS

In virtually every engineering application there is a need for measuring some physical quantities, such as forces, stresses, temperatures, pressures, flows, or displacements. These measurements are performed by physical devices called sensors or transducers, which are capable of converting a physical quantity to a more readily manipulated electrical quantity. Most sensors, therefore, convert the change of a physical quantity (e.g., humidity, temperature) to a corresponding (usually proportional) change in an electrical quantity (e.g., voltage or current).

Often, the direct output of the sensor requires additional manipulation before the electrical output is available in a useful form. For example, the change in resistance resulting from a change in the surface stresses of a material the quantity measured by the resistance strain gauges must first be converted to a change in voltage through a suitable circuit (the Wheatstone bridge) and then amplified from the milli volt to the volt level. The manipulations needed to produce the desired end result are referred to as signal conditioning. The wiring of the sensor to the signal conditioning circuitry requires significant attention to grounding and shielding procedures, to ensure that the resulting signal is as free from noise and interference as possible. Very often, the conditioned sensor signal is then converted to digital form and recorded in a computer for additional manipulation, or is displayed in some form. The apparatus used in manipulating a sensor output to produce a result that can be suitably displayed or stored is called a measurement system. Figure 15.1 depicts a typical computer-based measurement system in block diagram form.